Well, I have a 2nd hand Bluebird 28 with original carb and it's an absolute ripper. Very strong performance & no operating issues with the carb not shutting off etc. I'm very very pleased with it.

I also have an as yet un-run Brat 25 - to be fired up at a later date [8D]

Occasionally you do see Bluebird 46's pop up on the *bay so you might strike it lucky. It's also remotely possible that as the 46 size engines are reasonably generic in design, the heads of quite a few engines may fit the case of the Bluebird. Purely guess work on my part but if you are a tinkerer & a scrounger (oh joy) it may just work.